People keep mentioning pace and traditional wingers who will hug the touchline but these aren't the biggest problems. Yesterday we had Rashford upfront with Lingard and Mkhitaryan flanking him, how much more pace do we really need? And almost any good European side in the last decade or so deploys tactics with inverted wingers/forwards and it's the full backs who provide width in the final third.
Firstly, pace alone means nothing when your tempo is pedestrian in the first 2/3 of the pitch. We can get Dembele, Griezmann and Mbappe as our attacking trident and we'll get little improvement, if we don't also improve our transition game when we have possession. These, and the ones we currently have, are players who need room to run with the ball or run in behind and currently our biggest weakness is that we don't give them enough opportunities to make things happen while running with the ball. But this isn't about the wingers, it's about how we treat the ball in the build up phase of our game. Some people on here believed it was a tactical approach that aimed to take the best out of Zlatan but hindered our other attacking options but, as it proved, this is not the case.
As for the infamous width, the answer is (someone like) Valencia as a winger because he can stretch play on the right wing? Of course, it's not. What we so desperately need is players who are good with the ball and can create in tight spaces. These we lack and these type of players are the ones who make the difference at the highest level. Players with the ability to dribble, create, initiate and finish attacking moves. Mkhitaryan can play that role but he really needs to step up his game a gear or two. Martial could also do it but, whenever he is on the ball on the left, he seems completely unaware of his surroundings and he just tries to dribble everyone in front of him. Rashford, Lingard and Mata can offer very little in tight spaces and they need more room to operate, the first two because they lack the technical skills (Rashford has showed improvement lately though) and Mata because he can't push forward and be aggressive on the ball with opponents closing him down/marking him closely.
With a forward like Ibrahimovic, whose play making abilities are top notch, we can afford to have Rashford on the wings attempting runs in behind and looking to finish off chances others create for him. Even Griezmann, who has always thrived in systems with two forwards, will definitely benefit from an inverted forward playing closer to him.
But, whoever the main man upfront is next season, we need another player who can treat the ball neatly in under pressure and has good creative skills. Bernardo Silva is young, Portuguese and looks ready to take the next step in his career after the season he had with Monaco. Maybe Jose will look into him. But the winger alone won't solve all issues. Much will also depend on how Mourinho envisions the midfield and if he'll go into next season with Pogba-Herrera in front of the back four or not.